Optical disk data storage systems enable the storage of great quantities of data on a disk. The data is accessed by focusing a laser beam onto the data layer of the disk and then detecting the reflected light beam. The data is represented by changes in reflectivity.
An ablative WORM system allows the user to write data by making holes in the data layer of a blank optical disk. Once the data is recorded on the disk it cannot be erased. In an ablative WORM disk, the data layer is a thermally-sensitive, tellurium-based film that is coated on a glass or plastic substrate. The laser radiation enters from the back side of the substrate, passes through the substrate and heats the data layer. The tellurium-based material softens and the molten metal flows to form a hole. The holes typically represent the one bits in digitally-coded data, with the spaces between the holes representing the zero bits.
Commercially available ablative WORM disks are double-sided with two substrates bonded together, mounted on a metal hub and retained within a plastic cartridge. The two substrates are oriented with their respective tellurium-based data layers facing each other and separated by a substantially sealed air gap in the interior of the bonded structure. This form of disk construction protects the sensitive tellurium-based data layers from scratching during handling. Also, because there is no outer layer over the data layers to restrict the molten flow of the tellurium-based material, less laser power is required to ablate the data layers.
There are several disadvantages with this type of disk. There is only one laser and optical head assembly in ablative WORM optical disk drives. Because the disk can only be made in a way that is double-sided, the cartridge must be removed from the drive, turned over and reinserted into the drive to read or write data on both data layers. If used in an optical disk library the library must have a flipper mechanism to turn the disk cartridge over. This adds expense, complexity and size to the library. In stand-alone, non-library applications, the extra thickness of the disk and its cartridge also makes it more difficult to make a low profile optical disk drive, which is desirable for reduced cost and weight.
What is needed is a single-sided ablative WORM disk that does not need a separate protective cartridge, that can be handled without damage to the sensitive tellurium-based data layer, and yet can be easily written to at normal laser power.